This invention relates in general to inserts for earth boring drill bits and in particular to a surface finish on drilling inserts used on the cones of the bits.
A common type of earth boring bit a rolling cone type. The bit has a plurality of cones, normally three, that rotate about their own axes as the bit body is rotated about its axis. Each cone has cutting elements on the exterior that gouge and scrape the borehole bottom. For harder formations, tungsten carbide inserts are pressed into mating holes in the cones. Each insert has a cylindrical base that fits with an interference in the cone body. A cutting tip, which may have various shapes, protrudes from the base.
Tungsten carbide inserts are formed by pressing and sintering tungsten carbide particles in a matrix such as cobalt. After removal from the die, the base of the insert must be ground to the precise size. Conventional systems and methods of grinding of cemented carbide parts, such as drilling inserts, leaves relatively deep, wide scratches in their surface finish. This is due in part to the grinding wheel having diamond particles that stick up above the predominant grinding plane formed by the supporting matrix material. During use, the relatively soft matrix material tends to erode away faster than the larger, harder diamond particles, thereby exposing the diamonds to rough up the surface of the inserts.
Inserts with roughened surfaces require far more insertion force during installation, and have lower retention and durability during drilling. In addition, an insert with a low quality surface finish increases wall shearing of the steel body of the cone in the hole, which also diminishes retention.
Although inserts can be polished to a smoother surface finish, conventional polishing techniques are limited to treating surfaces that are substantially parallel to the polishing device. For example, the base of a drilling insert typically has a cylindrical barrel that tapers to a bevel on one end to facilitate insertion into the drill bit. When the insert is polished by conventional techniques, only the surface finish of the barrel is improved; the bevel remains unpolished. Unfortunately, the surface finish of the bevel has a significant impact on the performance of the insert.
A drilling insert has a cylindrical barrel, a bevel on one end, an interface therebetween, and a cutting tip opposite the bevel. The insert is forced into a hole in a drill bit under high pressure in order to overcome an interference fit therebetween. While being pressed into the hole, the bevel and interface contact the entrance to the hole and facilitate the transition of the barrel into the hole. A high quality surface finish on the bevel and interface are critical for enhancing the durability and retention of the insert in the drill bit.
The surface finish on the insert may be produced by polishing it with a diamond lapping film. The insert is rotated between three rollers as the lapping film passes over its surfaces. One of the rollers and the film conform to the axial profile of the insert as each element rotates, so that the interface, bevel, and barrel of the insert are simultaneously polished. Other surface finishing techniques, such as tumbling, also may be used to produce the desired surface finish. After polishing, a coating may be applied to the insert to retard corrosion. After coating, the insert is preferably polished again by the diamond lapping film process.